Amongst such detectors, the invention relates more particularly to those which use an appropriate detector to take advantage of an optical effect that relies on a relatively intense light pencil being reflected by some of the particles making up the smoke to be monitored, which smoke passes through a dark chamber containing said detector.
Given the intensity of the light pencil, if it were to be emitted continuously, the resulting electricity consumption of the device would be very large.
To avoid that drawback, proposals have already been made to emit said pencil in the form of light pulses of short duration t that are spaced apart from one another by identical much longer periods T, where the durations t are of the order or 1 millisecond or 100 microseconds, for example, while the periods T are of the order of 5 seconds to 10 seconds, detection then being performed on the basis of electrical response signals generated by the detector solely during the durations t, with the values of said signals being compared in turn with a value representing the above threshold so as to trigger the alarm if the threshold is exceeded.
To create said pulses of duration t, it is preferable to act as follows: a capacitor is charged during the periods T by means of a small electrical current, and at the end of each of said periods the capacitor is discharged during times t, and the current pulses of duration t generated in this way are applied to a light source, which current pulses give rise to light pulses of the same duration and forming the light pencil.
In general, in order to avoid untimely triggering of the alarm, as could be set off by the detector being suddenly illuminated, e.g. due to the device being scanned by an intense light beam itself created directly from a hand-held flashlight or indirectly by reflection of the sun on a glossy surface moving in view of the device, an alarm indicating a risk of fire is triggered only after verifying that the previously programmed trigger threshold has continued to be exceeded for a plurality of successive light pulses (see document EP-A-0 011 205).
If the spacing between said successive light pulses reaches or exceeds 5 or 10 seconds, the duration of such a safety check can run to half a minute or more, and that is prohibitive.